Be careful, the relationship between boost pressure and static compression is far from equal. Higher air density in the intake (boost) puts more oxygen in the chamber, and power potential is ultimately determined by the oxygen content of the mixture. Raising static compression improves combustion efficiency by squeezing the mixture harder, but it is no replacement for oxygen.

The maximum practical SCR (static compression ratio) is a factor of manifold pressure and fuel octane, all else being equal. That being the case, one has to think of it as a matter of 'how much compression can I safely run with at this boost pressure with this fuel'. If the SCR is such that it limits boost, then power will be limited. If the SCR is such that it allows for as much or more boost than will ever be used, then you're ok.

Turbo engines typically use moderate SCR (e.g. 8-9:1) because it allows more power to be generated with octane limited fuels. BMW limited SCR to around 8.5:1 with their turbo F1 motor that ran on pure toluene, but at 70psi of boost. That was enough to get 1350bhp from 1.5L, which is 900hp/L. That wouldn't have happened with higher SCR and less boost.

 

^^ See TedB's post....That said, if you are running a nicer fuel like e85, a slightly higher CR (say 10:1) should improve spool and transient response....

 

This is where I throw my thoughts in again on "spool". Higher compression does not improve spool, we have tested this back to back in a very larger change on a moderately sized turbo. The 3065 still spooled at 4k despite going from 9:1 to 11:1. It did make more power "all motor" which makes it more fun out of boost, but it is not the same as helping spool such as MIVEC can.

 

Talking compression is an interesting thread. But there are too many variables to give a definitive answer what is going to be the best compression for your setup.

Read on wiki about e85. it not only tolerates far higher compression than reg gas. it thrives on it. engines build at 10/1 on gas can be built to 15/1 on e85. problem for flex fuel vehicles is they have to be able to run on both fuels.

16g power records are abundant. the highest made on 9/1 is around 450-460. the 500+ records were set with 10-11/1 engines.

turbo size is crucial to this discussion. small turbos blow very hot air to begin with. max them out with boost and they might as well be blowing fire. small turbo not liking anything over 11/1 is no surprise. bigger turbos can for sure tolerate higher compression setups.

F1 cars dont add much to this discussion if anything at all. the build of an F1 engine is so far from the build of a street engine not really sure why it was even brought up. we dont run 17000 rpm with a 50mm stroke crank in an engine that makes 300tq tops. and the turbos we run are not happy at 5/1 pressure ratios either. we dont run tolulene at 90% either. that is one of the coldest burning most knock resistant fuels known.

what might be relevant is alcohol cars current day are running very high compression and very high levels of boost. local here worked for titan drag team. their supra engines are built at 14/1. they typically run 50-55 lbs boost but as high as 70psi has been run by accident without toasting the motor.

my recommendation would be to follow what has already be shown to work well. 10/1-11/1. experimenting with anything higher than that will be just that. an experiment.

 

Actually, the F1 example I quoted (circa 1988) is a stock block(!) engine with a redline of 11k rpm. Also, the PON rating of toluene is 114 octane (E85 is almost the same). When you look at the specifics, suddenly it sounds not far off from an engine we all know ...

 

Here in the real world........I think that's a good country song too.

The compression to boost comparison isn't even remotely close, throw that chart out the window, burn it or wipe a butt with it, it's not even close.

We've built a lot of high compression engines, remember the guy named John Shepherd, 7.7's at 191 in an AWD?

Project White was high compression, ran/runs on the red. It was high compression and stroked. The actual compression ratio on that engine was around 11:1 after all the calculations were taken, that is a custom built piston and we stock them.

I've got pistons for the 2 liter that are in the 11.5:1 range. These have been run at over 30 psi on nothing more than 93 octane with no problems and excellent results.

One thing I have found with higher compression is the off boost feel is improved, not as much as stroking an engine but it will make the same 2 liter feel much better and is easier to drive around. I do not however feel that the actual power gained from raising the compression substantially is even a medium increase in power, as a matter of fact I may go as far to say the power gains are hard to even measure. At one point I was convinced it was the way to go but with the continous testing we are always doing I am no longer convinced, as a matter of fact I am going the other way.

I think there is a lot to TedB's post about putting air into the cylinder and I don't usually agree with much he has to say You can fill the combustion chamber and cylinder with more aluminum or you can fill it with more air. There seems to be a happy medium and maybe more than anything power is dictated by compression and the design/shape of the piston top.

I am having another new batch of pistons made right now for a new project. Bad Bish has me re-thinking some things because of how well the new engine is working out, now to test just a piston "dome" change.

Personally, as a general rule from what I've seen on the 4G63, I'd say don't get too carried away on the compression. We've ran some of our best ET's and MPH's with about 8.8:1 to be honest and that is what our standard EVO piston sets are for our Stage 3 engines.

My 2 cents.

 

Exactly right. High static compression does not move more "air" (gas) through the engine. Higher compression simply compresses it more. This will not spool the turbo faster.

 

The formula for figuring "effective compression" is all over the internet. I have done several back to back compression only changes over the years. While I dont have any definitive data to show results there is no question in my mind higher compression makes more power all other factors being equal. The following equation assumes all other variables are equal. Biggest being the amount of airflow is constant for each boost level. Example comparing an 8/1 engine to 10/1 engine with 50lbs of air entering chamber. The 10/1 engine is going to make more power. its not even a debate. Its simple math.

that being said there are no constants in reality. raising the compression of engine increases its ability to make power on a given amount of air. but at same time it reduces the turbos ability to move the same amount of air due to their being less exhaust energy to power the turbine. Like I said their are way to may many variables to even begin to theorize what the right compression for every setup is.

http://www.dragsource.com/index.php?...s&calctoview=3

 

Higher compression increases power by improving combustion efficiency, and I don't think anyone here is challenging that. I've seen it offered from a credible reference that the change from 9:1 to 10:1 is worth about a 5% increase in torque, all else being equal. In an NA engine, this is a no-brainer, because a higher SCR always increases power so long as the fuel octane supports it, and so long as the piston crown doesn't impede flame propagation. The confusing factor in our situation is the presence of a turbo.

In a turbo motor, the basic thing to remember is that if you're limiting boost with static compression, it's a losing proposition. More boost trumps a higher SCR.

The term "compression ratio" is confusing when taken out of context - that is taken to mean anything other than the mechanical descriptor. The ultimate factor that determines torque output is 'dynamic compression', which is the cylinder pressure generated as a result of combustion. An 8:1 motor with greater boost will generate more dynamic compression than a 10:1 motor with lesser boost. If you can run MBT (if you have enough fuel octane) with the 10:1 motor with the same boost as the 8:1 motor however, the higher SCR motor has the edge in the 'dyno race'. If the higher SCR necessitates retarding the ignition timing however, even with the same boost, that may result in less dynamic compression than the lower SCR motor.

Higher compression tends to have a negative effect on spool because it results in a cooler exhaust charge. The counterbalance is greater off-spool torque, which tends to equalize the difference by accelerating the motor a little faster until it spools.

 

I'll just stick to reading in the future............

 

why? did you think no one minded your input? your text was awesome. very informative. for me I would rather test than theorize. then try to figure out why the results are the way they were. thats exactly what you do and its greatly appreciated. no one has the answers here. its an interesting discussion.

 

This has to be one of the best threads in quite sometime here, I will go through and read and re-read before I make a comment.

 

Please don't....read and post ....your experience and knowledge are awesome...On an aside, I wish you had an authorized shop out here in socal to install a Buschur motor ...

 

Here is a good question, maybe Mr Buschur can help with this one as most of us haven't a clue. Being that these high compression engines require high octane fuel and I use E100, not E85, not E98 but E100 uncut unblended... Does anyone know the octane limits of these fuels? Also how much is to much on Ethanol before having to mix race gas or switching to Methanol.

Also is there any real world results for 12.5 and up compression on a 4g63, I know there is a 14.1 compression evo some where out there... What ever happened to that car? There is a 13.1 compression 4g over seas, anyone know anything about these cars?

 

here a guy excited about e-85 and high compression. just made 533whp at 20psi on standard 35r. I dont remember ever seeing 9/1 motors do those numbers.

http://www.4g61t.org/forum/viewtopic.php?f=42&t=30736

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slowdsm, e85 let you get away with a lot of thing. rhys runs 14:1 on his car with e85. i have heard of 15:1 boosted on e85. 11:1 is actually too conservative. i think 12:1 is actually ideal for the street just in case you have to turn the boost down and run 89-93 octane pump gas until you get more e85. also with 12:1, you don't have to worry to much about clearances between the piston, valves, and spark plug. with a good tuning system, you don't have to run some rediculous boost to make nice horse power. traditional journal bearings and 360 degree bearing turbo are useful again because you not going to kill it because of the lower boost level.

the key to making power is to have an air/fuel mixture that is as dense as possible to create a combustion with the highest potential energy. people like low compression piston so they can have more control over ignition timing and boost to create more power. the draw back to low compression is you have to run more boost to make more power which. boost level above 25psi on a standard bearing will wear out the thrust bearing in the turbo pretty fast. at 30psi and above on a standard bearing turbo, you can expect a turbo rebuild pretty soon. at high boost on a low compression motor, you sill have to run race fuel or a fuel with a high octane value because the air/density is as high as a high compression motor/low boost setup.